Ink jet printing is a recent development in the art of applying identifying and decorative indicia to a base. In general terms, a fluid ink is forced, under pressure, through a very small orifice in an orifice block which contains a piezoelectric crystal vibrating at high frequency (50-100,000 vibrations per second) causing the ink passing through the orifice to be broken into minute droplets equal in number to the crystal vibrations. The minute droplets are passed through a charging area where individual droplets receive an electrical charge in response to a video signal, the amplitude of the charge being dependent on the amplitude of the video signal. The droplets then pass through an electrical field of fixed intensity, causing a varied deflection of the individual droplets dependent on the intensity of the charge associated therewith, after which the deflected drops are allowed to infringe on the base medium which is to receive the decorative or informative printed indicia. Apparatus suitable for carrying out the ink jet printing process is described in detail in U.S. Pat. Nos. 3,465,350 and 3,465,351, issued Sept. 2, 1969, and it is in connection with an apparatus and process such as are described in the aforementioned patents that the ink of the present invention is designed to function.
In addition to the requirement of proper wetting of the metal surface to be printed, the droplets of ink must adhere strongly to the surface, after application and drying, so that the printed matter is resistant to both physical rubbing or abrasive action and also is resistant to moisture. The ability of the ink to form and retain a desired image on a metal surface in the presence of moisture and the ability to resist removal by moisture is of great importance in this application because the metal can surfaces are generally damp before, during and after the printing operation.